If the wheels of a motor vehicle block during braking, they are then only still able to transmit a slight braking torque to the road. In addition, the wheels lose their lateral guidance force, so that the vehicle is no longer steerable. Even in vehicles having an integrated ABS system, the front wheels require a certain time period (wheel standstill time) in order to start up again after the blocking. During this time, they are not able to transmit any lateral force onto the roadway. The duration of the wheel standstill time is therefore especially critical for travel safety.
For the illustration of the dynamic processes in a usual ABS system we refer below to FIG. 1. It shows a braking system known from the related art, which is designed for ABS regulation, having a brake pedal 3, a brake booster 2 and a main brake cylinder 1 for producing brake pressure at a wheel brake 4. The brake circuit shown also includes an inlet valve 7, a wheel brake 4, an outlet valve 8 and a return pump 6. When brake pedal 3 is operated, a corresponding pressure is built up in the brake circuit, which acts upon wheel brake 4 via inlet valve 7, that is open in the normal state. In this state, outlet valve 8 is normally closed.
During an ABS regulation, the braking pressure acting upon wheel brake 4 is modulated using inlet valve 7 and outlet valve 8, the hydraulic fluid being able to be buffered in a buffer storage unit 9. A return pump 6 then conveys the hydraulic fluid coming from wheel brake 4 back in the direction of main brake cylinder 1. Thereby, the typical vibration of the pedal is created at brake pedal 3.
The duration of the wheel standstill time after the blocking of a wheel depends essentially on how rapidly the braking pressure acting on wheel brake 4 can be reduced. The speed of the pressure reduction is determined above all by the opening cross section of outlet valve 8, the accommodation capability of buffer storage unit 9 and the conveying power of return pump 6. For cost reasons, buffer storage unit 9 and return pump 6 are designed to be only as big as is absolutely necessary, so that, above all, these two components are limiting for the pressure reduction.
FIGS. 2a and 2b show the curve over time of wheel speed v and brake pressure p at the front wheels, for a usual ABS system, in a driving situation in which the wheels get into brake slip almost simultaneously. Since the front wheels behave essentially the same, only one characteristic curve 12 is shown here for the wheel speeds. In the driving situation shown in this instance, the vehicle first moves straight ahead at a speed v of about 20 m/s (see characteristic curve 11). After approximately 0.4 s, a braking process is initiated, by which brake pressure p rises to a value of about 140 bar. On account of a transition to a lower coefficient of friction (e.g. because of ice, wet pavement) after ca. 1.2 s, the front wheels begin to block; wheel speeds 12 of the two front wheels drop toward 0 m/s, as may be seen in FIG. 2a. Wheel speed 11 changes only slightly, however, since the vehicle having the blocked wheels at a low coefficient of friction continues to skid straight ahead.
As soon as the wheel slip of the front wheels exceeds a certain threshold value, a pressure is automatically reduced at wheel brake 4 by the opening of outlet valve 8. During the first phase of the pressure reduction (see FIG. 2b as of approximately 1.3 s) brake pressure 14 drops off at a maximum gradient. In this phase, the opening cross section of outlet valve 8 is the limiting variable for the pressure reduction. The hydraulic fluid flowing out of wheel brake 4 is temporarily stored in buffer storage unit 9. At about 1.6 s, buffer storage unit 9 is completely full (arrow 15). As of this time, the dynamics of the pressure reduction are determined by the conveying power of return pump 6. As may be seen in FIG. 2b, as of time 1.6 s, the pressure reduction runs in a substantially flatter manner. Only at approximately 2.5 s is a wheel start pressure of about 3 bar reached, as of which the wheel begins to start up again. The time span between the blocking and the renewed starting up of the front wheels (wheel standstill time) is designated in this case by reference numeral 13, and amounts to slightly more than 1 s. During this time span, the vehicle almost cannot be steered.